The weight of the rider is one force and gravity is another.
I wrote
The weight of the rider, is one force and gravity is another.
That is not the right place for the comma.
is it after is?
No.
You need a comma to separate the two independent clauses.
Your original sentence is correct in terms of conveying the idea that the weight of the rider and gravity are two separate forces. However, there are a few ways you can improve the sentence structure and punctuation. Here's a revised version of your sentence:
"The weight of the rider is one force, and gravity is another."
In this version, I added a comma before the coordinating conjunction "and" to properly separate the two independent clauses. This helps improve readability and clarity in the sentence.